1. Field of the Invention
This invention pertains to the calibration of an electronic output system for use with a color printer so that the final print obtained looks like an image obtained on a video monitor, both in color and tone scale parameters. More particularly, the invention pertains to a method for decoupling color and tone scale so that an adjustment of one parameter does not affect the other parameter.
2. Background Art
It is a common objective of a printing system to obtain a satisfactory match between an output print and an image produced of the print on a monitor. Electronic image processing is frequently used to adjust the color saturation and the contrast (tone scale) of the monitor image to the characteristics of the copy material or to accentuate the color saturation and contrast of the original image. Experience has shown that, for production of optimal images, several image parameters must be adjusted, or be capable of adjustment, including: the contrast (tone scale or brightness gradation), the neutral color balance, the color saturation, and the color tone or hue. Ordinarily, the adjustment of one parameter interacts with another parameter, i.e., the adjustment of one parameter is interdependent upon another parameter. When the gradation changes, for example, the color saturation changes as well.
One way of dealing with this interdependence is disclosed in U.S. Pat. Nos. 4,812,903 and 4,831,434. Initially, in accordance with known video techniques, RGB image signals are transformed into a brightness signal Y (luminance signal) and two color difference signals U, V (chrominance signals). Contrast (brightness gradation) is then corrected in the luminance channel and color saturation is separately corrected in the chrominance channel. Since changing the luminance continues to have an effect on color saturation, however, the '903 and '434 patents teach automatic correction of the color saturation in dependence upon the post-contrast processed luminance signal. More specifically, the correction is achieved by measuring the luminance signals Y.sub.1 and Y.sub.2 which occur, respectively, before (Y.sub.1) and after (Y.sub.2) contrast processing, and multiplying their quotient (.sup.Y 2/Y.sub.1) by each of the two chrominance signals U,V.
According to such a known technique, the color impression (color tone and color saturation) remains unchanged, even in the case of deliberate contrast changes in the luminance channel. This happens, of course, because there is automatic compensation for the increase in color saturation which accompanies a brightness reduction and for the decrease in color saturation which accompanies a brightness increase. Thus, despite the ability to change brightness gradation separately from color saturation, the independence achieved is incomplete since both channels must be modified for a deliberate change in one channel.